I think the key here is not so much her daughter's driving, but your husband's nerves. Will he annoy or make her nervous during her portion driving? I ask, because my dad would make comments when I first started driving that would have me second guessing myself. For this reason, as long as she is experienced with highway driving, I would say there should not be any problems with driving any portion of the trip, except for near Jellico or through Downtown Atlanta. Even then, the only reason I would not have her drive these are if he is likely to try and direct her and cause her an issue.
I know that when I was 22, myself and my 19 year old cousin helped my mom drive her Yukon from Ohio to Orlando with no problems. My cousin normally drove a small hatchback and I owned a Dodge Neon so that was a big difference in vehicles. I know I drove to Gatlinburg from Ohio before that. As long as she is experienced with driving on the highway, it should be fine. I know some people worry about driving through Cincinnati, but I drove myself and friends on a school trip at 17 with no problems.
Have you asked her if there are portions that she would be comfortable driving or that she would not want to drive?